Many electronic systems employ programmable gain amplifiers as a basic analog building block. An example of a conventional programmable gain amplifier (800) is illustrated in FIG.8. Programmable gain amplifier 800 includes an operational amplifier (AMP), three resistors (R81-R83), and two switches (S81-S82). The operational amplifier includes a non-inverting input terminal (INP), an inverting input terminal (INM), and an output terminal (AOUT). Switch S81 is connected between AOUT and node N81. Switch S82 is connected between AOUT and node N82. Resistor R81 is connected between node N81 and INM. Resistor R82 is connected between node N82 and INM. Resistor R83 is connected between INM and a circuit ground (GND).
In operation, operational amplifier AMP is configured to receive an input signal (e.g., IN) at the non-inverting input (INP), and provide an output signal (e.g. OUT) at the output terminal (AOUT) that is related to the input signal according to a gain scaling factor (X). The gain scaling factor (X) is determined by the impedance in the feedback path between the output (OUT) and the inverting input (INM).
Switches S81 and S82 are arranged to change the selection of the feedback components (resistors R81 and R82) to adjust the gain scaling factor (X). The selected feedback component is resistor R81 when switch S81 is closed, while the selected feedback component is resistor R82 when switch S82 is closed. The relationship between the input signal (IN) and the output signal (OUT) is determined by the feedback components and the switches as follows: OUT=IN*(1+X), where X is determined by the feedback components. In one example, X=R81/R83 when switch S1 is closed and switch S2 is open. In another example, X=R82/R83 when switch S2 is closed and switch S1 is open.